Steam generators of pressurized water nuclear reactors contain a nest of tubes which are bent into U shape and whose ends are fixed in a tubular plate of great thickness and lie flush with one face of this tubular plate constituting the primary face coming into contact with the pressurized water. Beneath the tubular plate the distribution of the primary pressurized water in the tubes and the recuperation of this primary water are effected inside a water tank which has a hemispherical shape and is divided into two parts by a vertical partition. This water tank has an opening for the admission of the pressurized water and an opening for the evacuation of water which has circulated in the tubes situated on each side of the partition. This water tank also has at least one manhole for the inspection, maintenance and repair of the steam generator.
Steam generators of pressurized water nuclear reactors must operate over very long periods of time, and it may be necessary to make repairs to some tubes which may have cracked after a certain period of use in the steam generator. In order to locate with certainty the tubes which have become cracked, it may be necessary to introduce in these tubes an eddy current or ultrasonic probe, which will require a minimum diameter for its passage through the tube.
Tube ends near the primary face of the tubular plate may be deformed and bent inwards through the action of impacts of migrant bodies or particles transported by the pressurized water during its circulation at high speed in the vessel of the nuclear reactor and in the steam generator. The ends of the tubes very slightly project from the primary face of the tubular plate, and these projecting ends are liable to become deformed. The ability to introduce a probe into the tube may then be reduced.
On the other hand, it is possible either to repair cracked tubes by a lining operation, or to put the cracked tube out of action by sealing its end lying flush with the tubular plate with the aid of a stopper of a special design.
Finally, when the repair or stopping operations have been completed, it is necessary to check that these operations have been correctly carried out.
For these multiple operations of checking, repair or stopping, it is necessary to introduce tools or auxiliary parts into the tube. The ability to introduce such equipment is reduced when the tube ends are deformed.
Before a checking or repair operation on the tubes of a steam generator, it is therefore necessary to know the shape and dimensions of the hole for passage through the tubes in the proximity of the primary face of the tubular plate.
In steam generators which have been in operation, it is scarcely conceivable to make this inspection directly and visually, because the water tank of the steam generator will have undergone heavy radioactive contamination.
A simple examination by a video system would not provide sufficiently accurate information, and it would be necessary to devote a very long time to the optical examination of the thousands of steam generator tubes inside the water tank.
The use of photographic images of the different parts of the tubular plate and of the ends of the tubes would likewise not make it possible to ascertain with great accuracy the shape of the tubes and the passage diameter inside them.